The present invention describes a method and apparatus for use in producing a structural wood particle board. The board has one or more layers of wood elements or strands oriented into parallelism with each other. A product of this type and the merits of orienting the wood elements thereof are described in U.S. Pat. No. 3,164,511.
The method and apparatus described herein are improvements over U.S. Pat. No. 3,478,861, which teaches stationary, parallel plates arranged in vertical planes and spaced from each other at distances less than the average length of the strands above a support. The plates have movable elements adjacent to their upper edges, such as segments of a cord in tension or metal caps, which move along these upper edges parallel to each other but in opposite directions from elements immediately adjacent thereto. The vertical plates, being stationary, require strands with closely controlled geometry. A single strand having a width greater than the gap between two plates, a partly broken strand, or too many strands at a given time and location may cause a gap to fill up, thereby interrupting the process of felting or depositing the strands on a moving support therebelow. This limitation requires careful screening of the strands and formation of an oriented mat of strands at a relatively slow rate. Furthermore, if orientation of the strands transversely to the movement of the support is desired, only a relatively slow speed of the movable support is possible, and the lower edges of the plates must be at a substantial distance from the felted mat, which allows some of the strands to disorient again,
Other disclosures relating to the problem of orienting wood strands include U.S. Pat. Nos. 3,114,431 and 3,721,329. Both of these patents, however, fail to disclose or suggest either an apparatus or a method which orients the strands in the same way or as efficiently as is capable with the practice of the present invention hereinafter described.
The apparatus for producing an oriented strand mat must be capable of handling the large volume of strands per unit of time which is the standard in the present-day particle board industry because of the high investment cost of such a plant. For instance, a typical 500 tons per day plant requires mat formation at the rate of almost 800 lbs. of strands per minute in a 22-hour day operation. Generally, in many plants, there are four forming heads; therefore, four orienting mechanisms would be required. Each orienting mechanism must therefore orient about 200 lbs. of strands each minute. It must do so without interruption or occasional strand congestion. Orienting means, such as vertical plates or belts, should be relatively thin. If thick members would be used when orienting parallel to support movement, the resultant mat will show ridges, but more importantly, the volume of strands, hereafter referred to as through-put, which can be oriented, will decrease as the ratio of gap space to plate thickness decreases. This is particularly true if the distance from plate to plate must be small due to shorter strand length. Stationary plates with moving metal caps on their upper edges, as in U.S. Pat. No. 3,478,861, provide an example where the thickness of the plates is increased at their upper edge by a wider moving cap or channel. This additional width is being taken away from the gap between plates and thereby decreases the through-put of the apparatus.